In low-IF receivers, a radio-frequency (RF) signal is mixed down to intermediate frequency (IF) before data is recovered from the received signal. Low-IF receiver architecture is popular due to its inherent immunity to DC offsets and noise. An analog low-IF receiver often includes a quadrature down-converter that initially down-converts an incoming analog frequency-modulated (FM) signal to two ideally matched channels before further converting data carried by these channels into digital data.
In particular, an input analog FM signal may be represented as a combination of two sinusoids that are 90° or a quadrature phase apart from each other. This separation may be accomplished by first splitting the signal equally between two channels using, for example, a flow splitter. The resulting signals are then each mixed with signals produced by two local oscillators. The two local oscillator signals have a quadrature (90°) phase difference between them which separates the input signal into an in-phase (I) component and a quadrature-phase (Q) component, where the in-phase component is often associated with information carried by a cosine sinusoid and the quadrature component is often associated with information carried by a sine sinusoid.
This operation of mixing an input signal with the two quadrature phase oscillator signals alters the frequency content of the input signal. For example, mixing a signal m(t) with two oscillator signals sin(ωLOt) and cos(ωLOt) is mathematically equivalent to multiplying, in time domain, the signal m(t) with the complex sinusoid ejωLOt=cos(ωLOt)+j sin(ωLOt). This will produce a resulting signal m(t)ejωLOt in which its frequency content is shifted by ωLO in comparison to m(t).
The mixed outputs may then be low-pass filtered to generate low-IF analog “I” and “Q” signals before being sampled and converted to digital values using analog-to-digital converters (ADC) on the respective channel paths.
The parallel nature of the two channels requires the channels to be closely matched for accurate I/Q measurements. In addition, the phase difference between the two channels must ideally be 90° at all frequencies. Often times, however, these requirements are not met due to the circuit's susceptibility to I/Q channel imbalance. One source of I/Q channel imbalance may be the flow splitter used to divide an incoming analog signal equally between the “I” and “Q” paths which may introduce phase and gain differences between the two paths. Another source of imbalance is the quadrature phase splitter used to generate the “I” and “Q”-oscillator signals which may introduce a non-orthogonal phase difference between the two signals. Furthermore, there may be differences in conversion losses between the output ports of the “I” and “Q”-channel mixers. In addition, filters and ADC's on the “I” and “Q” paths may not be perfectly matched. Theses complications will be referred to herein as “I/Q mismatch.” I/Q mismatch often times degrade the performance of a communication system. While various components of a communication system may be carefully designed and manufactured, I/Q mismatch can nevertheless occur due to temperature dependencies or other environmental phenomena affecting the quality of the components.
Hence, it would be desirable to provide a method and implementation for accurate I/Q mismatch compensation and calibration in an analog FM receiver.